Balanced modulator circuit



April 29, 1941. R. B.' J. BRUNN D MoDgLAer-KEUIT Filed March s. 1939 M"Y BALANCE 2 n@ ww Anw L im. m m. Mwah@ INVENTOR ROBERT B. J. BRUNNElx/Mm ATTORNEY Patented Apr. 29, 1941 Robert B. J. Brunn, New .York, N.Y., assignor to Hazeltine Corporation,v a corporation of Dela- WareAppucauun March 3, 1939, serial No. 259,546 5 Claims. (Cl. 179-1715)This invention relates generally to balanced modulator circuits andparticularly to balanced modulator circuits from which it is rdesired toderive onlyr heterodyne-frequency signals of two signal-input sources.While the invention is of general application, it is of particularutility in television signal-translating apparatus.

Balanced or push-pull modulators have been utilized in prior artarrangements to derive heterodyne-frequency signals from twosignal-input sources. In push-pull modulators of-the prior art, it hasbeen possible to balance the arrangement from one of the sets of inputterminals to the output terminals, whereby signals of thefrequency ofthe input to said one of said sets of input terminals do not appear atthe output terminals ofthe modulator. If only the heterodynefrequencysignals are desired, however, it is necessary to provide some othermeans for eliminating signals of the frequency of the other input signalfrom the output of the modulator. Selective circuits or lters havecommonly been utilized for this purpose. However, in order that thedesired signals of heterodyne frequencies may be selected in thismanner, it is necessary that they differ substantially in frequencyfromthe signals of input frequency which are to be rejected by the selectivecircuit. This relationship is sometimes difficult or impossible toprocure. Furthermore, the selector circuits required in sucharrangements add to the cost and compleXity of the modulator andassociatedcircuits.

It is, therefore, an object of the invention to provide a balancedmodulator circuit which is not subject to the above-mentioneddisadvantages Y of the arrangements of the prior art.

It is a further object of the invention to provide a balanced modulatorin which only signals of heterodyne frequencies of two signal inputsthereto appear at the output terminals thereof.

It is still another object of the invention to provide a modulatorcircuit in which signals of the frequencies of each of vtwo inputsourcesare balanced with respect to the output circuit of the modulator.

In accordance with the invention, a balanced modulator comprises twosets of input terminals adapted to have signals rapplied thereto, a setof output terminals, and means for developing signals of heterodynefrequencies across the output 'terminals and balancing out at the outputterminals the signals applied to both of the sets of input terminals.The modulator includes two pairs of vacuum tubes each tube having inputand output sets of electrodes together with meansforV coupling theoutput electrodes to the output terminals, the output electrodes of onepair of tubes being coupled to the output terminals with a polarityopposite to those of the other pair.

The modulator also includes means for connecting one of the sets ofinput terminals in pushpull relation to input electrodes of both of thepairs of tubes, the push-pull connections to the pairs of tubes being oflike polarity with respect to corresponding tubes 'of each pair. Themodulator additionally includes means for connecting the other of thesets of input terminals in pushpull relation to input-electrodes of bothof the pairs of tubes,` the push-pull connections of such other set ofinput terminals .to the pairs of `tubes being of Vopposite polaritieswith respect to corresponding tubes of each pair. Further, in accordancewith a preferred embodiment of the invention, the modulator is.incorporated in a modulated--carriersignal transmitter including anoutput circuit, aw-source or high-frequency oscillations, and a sourceof modulated-carrier frequency signals. In this embodiment of theinvention, one of the sets of input terminals of the modulator iscoupled to the source of high-frequency oscillations,the other of thesets of input terminals is coupled tothe source of modulatedcarriersignals, and the output terminals of the modulator .arercoupled to theantenna circuit.

. Preferably, the input terminals, the output terminals, andtheabove-referred-to pairs of vacuum tubes have a common neutral which isgrounded.

For a`better understanding of the invention together with otherand-further objects thereof, reference is `had to the followingdescription taken in yconnection 'with the accompanying drawing, and itsscope will be pointed out in the appended claims. V A

Fig. 1 of the drawing is a circuit diagram, partlyy schematic, of anVembodiment `of the invention VincorporatedV as amodulator in a'modulated-carrier televisionsignal transmitter, while Fig. 2 is acircuit diagram of a'modulator similar A to that of Fig.- 1 utilizingvacuum tubes having a single control electrode. K v

`4Referring nowto Fig. l ofjthe drawing'and neglecting for the momentthe operation of vthe modulator ycircuit of the invention presently tobe described, there is illustrated Va circuitfdiagram of amodulated-carrier 'television signalv transmitter inwhch theconventional portions are indicated schematically' since, Y per se,vthey form no part'of the invention. 1 This'transmitter comprises tinone signal-translatingwchannel thereof, couquency combining amplifierI Whereinthey arev l further amplified, the two above-mentioned channelscomprising common units in the remaining portions of the transmitter.The signal output of intermediate-frequency amplifier IS is coupled toinput terminals |1 of the modulator circuit i3 of the invention,presently to be described. There is also coupled to the set of inputterminals I9 of modulator circuit I8 aysource of locally-generatedoscillationst. Connected to the output terminals 2| of modulator 18, inthe order named, are a balanced transmission line 22, a radio-frequencyamplier 23, a second balanced transmission line 24, and a double-tantenna 25, 25.

Neglecting the momentY the portions of the system comprising the presentinvention, the operation of the circuit just described is wellunderstood in the art and a detailed description thereof is believed tober unnecessary., In brief, however, video-frequency signals, providedby source I3, are utilized to modulate a source of locally-.generatedoscillations and amplified in oscillator-modulator andintermediate-frequency amplifier I. The intermediate-frequency output ofunit l! is further, amplified and the carrier wave and la', singlesideband thereof are selected in amplifier and s ingle-sideband selector|2. Audio-frequency signals, provided by source I3, are utilized tomodulate a source ,of locally-generated oscillations inoscillator-modulator and intermediate-frequency amplifier I4 to providean audio-modulated intermediate-frequency signal which is furtheramplified in amplifier-selector |5. The outputs of units I2 `and I5 arefurther amplied and combined in intermediatefrequency combiningamplifier I6 and applied to modulator I3 wherein theheterodyne-frequency signals of the output of unit I6 and oscillator 20yare provided at output terminals 2 I. The videoand audio-modulatedcarrier-frequency signals so provided are transmitted over balancedtransmission line 22 to radio-frequency amplifier 23, wherein they arefurther amplified, and transmitted over balanced transmission line 24 todoublet antenna 25, 25 for radiation in a manner well understood in theart.

Coming now to the portion of the system comprising the presentinvention, there is shown a modulator circuit including means for.developing signals of heterodyne frequencies across the outputterminals 2|, 2| and balancing out atthe output terminals thesignalsapplied to both of the sets of input terminals |1, |1 and I9, I9,this means comprising four vacuum Atubes 30, 3|, 32, and 3 3 each havingtwo sets of input electrodes and one set of output electrodes. Means isprovided for coupling the output electrodes of the pair of tubes and 3|with one polarity to output terminals 2| and the output electrodes yoftubes 32 and 33 to output terminals 2| with the opposite polaritycomprising 'an 'attenuator inluding a mid-'tapped inductance '31|variably inductively coupled to inductance 35. A set of input electrodesof the tubes of each of the pairs 30, 3| and 32, 33 is coupled to inputterminals |1 with opposite polarity for balanced operation with respectto the output terminals 2|, and the remaining set of input electrodes ofthe tubes of one pair 3u, 3| is coupled to input terminals I9 withopposite polarity to the input electrodes of tubes 32, 33 of the otherpair, also for balanced operation with respect to output terminals 2|'.An attenuator comprising an inductance 36 variably inductively -coupledto a mid-tapped inductance 31 is provided for coupling input terminalsI9 to one set of input electrodes of each of tubes 3-33, while `anattenuator comprising an inductance 33 variably inductively coupled to amid-tapped inductance 39 is provided for coupling input terminals I1 tothe others of the sets of input electrodes of tubes 30-33. 'Ihemid-points of inductances 34, 31, and 39 are grounded, the cathodes oftubes 33 and 32 are grounded through a common source of grid bias 40,and the cathodes of tubes 3| and 33 are grounded through a common sourceof grid bias lil.V Thus, the attenuator 36, 31 comprises balanced meansVincluding the common grounded neutral for yconnecting the set of inputterminals |9, |9 in push-pull relation to one set of input electrodes ofboth of the pairs of tubes 3-33, inclusive, the push-pull connections tothe pairs of tubes being of like polarity with respect to correspondingtubes of each pair. Likewise, the attenuator 33, 3S comprises balancedmeans including the common grounded neutral for connecting the set ofinput terminals I1, |1 in pushpull relation to the other set of inputelectrodes of both'of the pairs of tubes 30-33, inclusive, the push-pullconnections of the latter set of input terminals to the pairs of tubesbeing of opposite polarities with respect to corresponding tubes of eachpair.

In considering the operation of modulator circuit I8, it will be assumedthat a signal is applied at terminals I9 to provide a radio-frequencyvoltage across inductance 31 of the instantaneous polarity shown andthat modulated intermediatefrequency signals are applied at inputAterminals to provide a voltage across inductance 39 of theinstantaneous polarity shown. It will be seen that a negative voltagefrom inductance 31 is applied to an input electrode of tube 3| and to aninput,V electrode of tube 32 and, as the output electrodes of v.thesetwo tubes are connected to output terminals '2| with opposite jpolarity,that substantially no `resultant signal Voltage of the frequency of thesignal applied to terminals I9 is developed at the output terminals 2|by the two tubes. Similarly, a positivepotential from induotance 31 isapplied 'to an input electrode of tube 30 and to an input electrode oftube 33 and, as these two tubesv are coupled to output terminals `2|with opposite polarity, substantially nofrersultant signal voltage ofthe lfrequency of the signal input to terminals I9 appears at the outputterminals 2| under these conditions. Similarly,signals of the frequencyof the input to terminals I1 are substantially balanced with respect to`ou'tput'terminals 2| and substantially do not appear at the outputterminals '2|' of modulator I8. i

However, with respect to signals of heterodyne frequencies of thesources applied at input terminals I9 and input terminals |1, it Willybe seen that vthe components developed by .the several tubesareadditive with respect .tothe output terminals 2|. Thus, under theconditions assumed,

a negative potential is' applied from inductance 31 toV an inputelectrode of tube 3| and a negative potential is also appliedfrominductance .39 to another input electrode of tube 3|. y Therefore, thesign of` the heterodyne-signal component is positive asindicatedadjacent the anode of tube 3l, Similarly, the sign of theheterodyne-signal output of tube 30 is positive for the reason that apositive potential, is appliedto one orf its input electrodes frominductance t 31 and"v a positive potential is applied to another inputelectrode from -inductance 39. Upon consideration, it will also be seenthat, under the conditions assumed, the sign of the heterodyne signalsprovided by tubes 32`an`d 33 are negativeas indicated adjacent theanodes thereof. It i`s`, therefore, apparent that signals of heterodynefrequencies ofthe two signal sources applied, respectively,V atterminals l1 and .I9 are additive` inthe output circuit vrof modulatorI8 and appear 2| ofthe modulator. v Y

It will be seen that the actionof thetransmitterillustrated is, withvrespect to either'the video-frequency channel or the` audioffrequencychannel, simply the reverse of that of a conventional superheterodynereceiver. Furthermore, it is apparent that, under these conditions,oscillator and radio-frequency amplifier 23 may be tunable andunicontrolled to provideya tunable transmitter for transmission at anyselected carrier frequency within a predeterminedv range of frequencies.1

While it is not'intended 'to limit the several units of the invention tooperation at any particular frequency or within any particular frequencyrange, there follow two groups of recommended frequencies or frequencyranges for each of the uni'ts which are particularly suitable for latthe output terminals ing tubes of each pair, and means for connectingthose skilled in the art that various changes and modifications may bemade therein without departing from the invention, and it is,-therefore, aimed in the appended claimsv to coverall such changes andmodiiications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A balanced modulator comprising, two sets of input terminals adaptedto have signals ape plied thereto, a set of output terminals, and meansfor developing signals of heterodyne frequencies across said outputterminals and balancing out at said output terminals the signals appliedto both of said sets of input terminals, said means including two pairsof vacuum tubes each tube having input and output sets of electrodes,means for coupling said output electrodes to said output terminals, theoutput electrodes of one pair of said tubes being coupled to said outputterminals with a polarity opposite to those ofthe other pair, means forconnecting one of said sets of input terminals in push-pull relation toinput electrodes of both of said pairs of tubes, the push-pullconnections to said pairs of tubes being of like polarity with respectto correspondthe other of said sets of input terminals in pushamodulated-carrier television signal transmitter: Y

In Fig. 2 there is illustrated a modification of the circuit of Fig. 1in which the modulator tubes -33, inclusive, comprise a single controlelectrode; that is, each tube has only one set of input electrodes.Similar circuit elements in the two iigures have identical referencenumerals. In Fig. 2 the voltage supplied from transformer 36, 3l isapplied directly to the control electrode of the vacuum tubes 30-33,inclusive, and secondary winding 3l has coupled between its mid-tap andground a source of bias potential 5l). The input voltage suppliedthrough transformer 38, 33 is connected in the cathode circuit of eachof the tubes and a source of bias potential 5I is provided between themid-tap of winding 39 and ground. The operation of the circuit of Fig. 2is very similar to that of Fig. 1 and no further description thereof isdeemed to be necessary.

While there have 'been described what are at present considered to bethe preferred embodiments of this invention, it will be obvious to pullrelation to input electrodes of both of said pairs of tubes, thepush-pull connections of said other set of input terminals to said pairsof tubesbeing of opposite polarities with respect to corresponding tubesof each pair.

2. 'A balanced modulator comprising, two sets of input terminals adaptedto have signals applied thereto, a set of output terminals, and meansfor developing signals of heterodyne frequencies across said outputterminals and balancing out at said output terminals the signals appliedto both of said sets of input terminals, said means including two pairsof vacuum tubes, each tube having two sets of input electrodes and a setof output electrodes, means for coupling said output electrodes to saidoutput terminals, the output electrodes of one pair of said tubes beingcoupled to said output terminals with a polarity opposite to those ofthe other pair, means for connecting one of said sets of input terminalsin push-pull relation to one of said sets of input electrodes of both ofsaid pairs of tubes, the push-pull connections to said pairs of tubesbeing of like polarity with respect to corresponding tubes of each pair,and means for connecting the other of said sets of input terminals inpush-pull relation to the other of said sets of input electrodes of bothof said pairs of tubes, the push-pull connections of said other set ofinput terminals to said pairs of tubes being of opposite polarities withrespect to corresponding tubes of each pair.

3. A balanced modulator comprising, two sets of input terminals adaptedto have signals applied thereto and having a common neutral, outputterminals having the same common neutral, and means for developingsignals of heterodyne frequencies across said output terminals andbalancing out at said output terminals the signals applied to both ofsaid sets of input terminals, said means including two pairs of vacuumtubes each tube having input and output sets of electrodes, means forcoupling said output electrodes to said output terminals, the outputelectrodes of one pair of said tubes being coupled to said outputterminals with a polarity opposite to those of the other pair. meanscomprising said common neutral for connecting one of said sets of inputterminals in push-`pull relation to input electrodes of both of saidpairs of tubes, the push-pull connections to said pairs of tubes beingof like polarity With respect to corresponding tubes of each pair, andmeans comprising said common neutral for connecting the other of saidsets of input terminals in push-pull relation to input electrodes ofboth of said pairs of tubes, the push-pull connections of said other setof input terminals to said pairs of tubes being of opposite polaritieswith respect to corresponding tubes of each pair.

4. In a modulated-carrier signal transmitter including an outputcircuit, a source of highfrequency oscillations and a source ofmodulated carrier-frequency signals, balanced modulator means fordeveloping heterodyne frequencies across said output circuit andbalancing out at said output circuit said modulated signals and ignals`of the frequency of said high-frequency source, said means comprising,two pairs of vacuum tubes each pair having input and output electrodes,means for coupling said output electrodes to said output circuit, theoutput electrodes of one pair of said tubes being coupled to said outputcircuit with a polarity opposite to those of the other pair, means forconnecting said source of high-frequency oscillations in push-pullrelation to input electrodes of both of said pairs of tubes, thepush-pull connections to said pairs of tubes being of like polarity withrespect to corresponding tubes of each-pair, and means for connectingsaid source of modulatedcarrier frequency signals in push-pull relationto input elee'ztodesl of both of said pairs of tubes, the push-pullconnections' of said last-mentioned source to said pairs of tubes beingofV opposite polaritiesv with respect to corresponding tubes ofeach-pair'. A

5. In a carrier-frequency signal transmitter including a balanced outputcircuit, abalanced source ofl high-frequency oscillations, and abalanced source of modulating signals, balanced modulator means fordeveloping heterodyne frequencies across said output circuit andbalancing out atsaid output circuit said modulated signals and signalsof the frequency of said high-frequency source, said means comprisingtwo pairs of vacuum tubes each tube having input and output electrodes,balanced means for coupling said output electrodes to said outputcircuit, the output electrodes of one pair of said tubes being coupledto said` output circuit with a polarity opposite to those of the otherpair, balanced means for connecting said source of high-frequencyoscillations in push-pull relation to input electrodes of both of saidpairs of tubes, the push-pullconnections to said pairs of tubes being oflikeY polarity With respect to corresponding tubes of each pair, andbalanced means for connecting said source of modulating signals inpush-pull relation to input electrodes of both of said pairs of tubes,the push-pull connections of said source of modulating signals to saidpairs of tubes being of opposite polarities with respect tocorresponding tubes of each pair.

ROBERT B. J. BRUNN.

